Today, February 3, marks the 100th anniversary of the ratification of the 16th Amendment, which gave Congress the power to create an income tax. Shortly thereafter, Congress enacted the Income Tax Law of 1913, 38 Stat. 114 (Oct. 3, 1913) -- the first of many! (The income tax provisions start on page 166)
The 1913 Form 1040 return was the picture of simplicity and was only 4 pages, including instructions. An annual exemption of $3,000 (the equivalent of $70,000 today!) enabled most Americans to escape the 1%--6% tax.
The legislative history of the Income Tax Law of 1913 is available in Seidman's legislative history of federal income tax laws, 1938-1861, which the Library has in its print collection. An extensive collection of pre-compiled legislative histories of federal tax laws is available on Hein Online in the "Taxation & Economic Reform in America Parts I & II, 1781--2012" library.